1. Field of the Invention
The present invention relates to a sensor unit for assay in utilizing attenuated total reflection. More particularly, the present invention relates to a sensor unit for assay in utilizing attenuated total reflection, in which a sensing surface can be positioned precisely relative to a flow channel for flow of a sample fluid.
2. Description of the Related Art
An assay apparatus for assay in utilizing attenuated total reflection is used for various kinds of studies in a biochemical field or the like, for example to study interaction of protein, DNA and various biomaterials, and to select candidate drugs by screening. Also, the technique is useful in the fields of the clinical medicine, food industries and the like.
A surface plasmon resonance (SPR) sensor is known as an assay apparatus in utilizing attenuated total reflection. Surface plasmon is a term to mean the compressional wave created on the surface of the metal and included in plasmon as quantized expression of the compressional wave. Free electrons in a metal vibrate to generate the compressional wave.
U.S. Pat. No. 5,313,264 (corresponding to JP-B 3294605) discloses an SPR assay apparatus in which an optical system of Kretschmann configuration is used for incidence of light to the metal film. According to the Kretschmann configuration, the thin film/dielectric interface of the metal film is fitted on a prism, which condenses light and directs the light to the thin film/dielectric interface in a manner conditioned for total reflection.
Upon the total reflection created on the metal/dielectric interface, a small component of the light passes through the metal film without reflection, and penetrates to the sensing surface. A wave of the penetrating component is called an evanescent wave. Surface plasmon resonance (SPR) is created when frequency of the evanescent wave coincides with that of the surface plasmon. In response to this, intensity of the reflected light attenuates remarkably. In the assay apparatus, the attenuation in the reflected light reflected by the metal/dielectric interface is detected, to recognize creation of the SPR on the sensing surface.
A sample or biomaterial, such as protein and DNA, are handled as sample fluid for the purpose of preventing deactivation or modification due to drying. The sample fluid contains biomaterial and fluid medium, examples of which include pure water, physiological saline water, liquid buffer and the like. U.S. Pat. No. 5,313,264 discloses the assay apparatus for analyzing interaction of the sample. The assay apparatus has a flow channel for flowing and introducing the sample fluid on to a sensing surface. Also, a linker film is overlaid inside the flow channel to have the sensing surface, for immobilizing the sample. Ligand fluid is introduced to the flow channel for immobilizing the ligand on the sensing surface. After this, analyte fluid is introduced for contact of the analyte and the ligand, to assay the interaction between those.
The assay apparatus of U.S. Pat. No. 5,313,264 includes an assay stage where a prism and a flow channel are disposed. A sensor unit of a chip type is positioned on the assay stage for the measuring step, the sensor unit including a glass support and a thin film of metal overlaid thereon.
A problem in the immobilization is in that time required from the introduction of the ligand until completion of the immobilization is as long as one (1) hour or so. This is remarkably longer than the measuring step for the assay which can take only a small number of minutes. As the assay stage of U.S. Pat. No. 5,313,264 is used for both of the sample immobilizing flow and the measuring step for the assay, the assay stage must be occupied for the immobilization during the assay. Other sensor units cannot be set on the assay stage. This is a problem of delay in the operation on the assay stage.
It is conceivable to immobilize samples for all of plural sensor units before assay of those, for the purpose of raising efficiency. However, there is a drawback in that the sensing surface will be dried to deactivate the samples, or will receive impurity stuck thereto, particularly after storing the sensor units with immobilized samples. No assay with precision is possible.
In view of this, an SPR sensor unit is conceived, including a flow channel block, a prism and a retaining block. The flow channel block has a flow channel. The prism is overlaid with a thin film of metal. The retaining block keeps the flow channel block positioned on the prism by setting the flow channel at the thin film.
According to the conception, the flow channel and the prism are provided in the sensor unit itself. The immobilization and the assay are made in simultaneously sequences for plural sensor units, so as to raise efficiency in the operation. Also, the disposition of the flow channel allows preserving of the sensor unit with the sample fluid as storage after the immobilization. Drying of the sensing surface can be prevented.
The flow channel block is formed from rubber or other elastic material for tight contact with the thin film of metal. A retaining block presses the flow channel block against the thin film and kept deformed resiliently. However, the width of the flow channel is as small as one (1) mm. No known technique can eliminate offsetting of the flow channel relative to the sensing surface, because assembly with sufficient precision is extremely difficult.
If the linker film with the sensing surface is offset from the flow channel, an area defined by facing of those changes. Thus, an amount of immobilized ligand changes to influence precision in the measuring step. Enlargement of the linker film may be conceivable in comparison with the flow channel. However, regents or other materials for forming the linker film with the sensing surface are considerably expensive. An unwanted large area of the linker film with the sensing surface is inconsistent to a manufacturing cost which should be reduced to a low level.